The Ontario Fire Marshal’s Office is investigating a fire that destroyed a commercial building and sent thick black smoke billowing through downtown.

Flames were shooting through the roof of the building on Thames Street when Chatham-Kent firefighters responded at about 1 p.m. Saturday.

Crews from three stations battled the blaze that forced police to close roads and evacuate some homes. Residents also were warned to close their windows and doors.

Chatham-Kent police said investigators from the fire marshal’s office were expected to be on the scene Sunday.

The building at 61 Thames St. near the Thames River housed Church’s Army & Outdoor Military Surplus, as well as The Hot Tub Men.

Fire officials and police warned residents to stay away from the scene, and to close their windows and doors if they live in the neighbourhood to prevent exposure to smoke.

“If you are inside your home, please close all windows/doors and shut off (the) HVAC,” the Chatham-Kent fire department said in a media release.

There was no word on a possible cause or damage estimate but police said the building was destroyed. It is also not known which substance caused the noxious smoke.

A large group of onlookers stood by initially, but police eventually moved everybody out of the area as the smoke began to intensify.

Roads were closed several blocks away and some people were asked to evacuate.

Chatham-Kent police Sgt. Paul Pomajba said two employees and three customers were in the army surplus store at the time and able to escape.

“They heard the noise crackling and they called 911, and got out safely,” he said at the scene.

Michael Bondy, who lives nearby on Victoria Avenue, said he smelled smoke but wasn’t expecting what he saw. “Little did I know it would be a visible inferno,” the Chatham councillor said. “We can feel the heat from (here). . . I’ve never seen flames like this.”

Dave White, a Kent Street resident, closed his windows to protect himself and his pets.

“All I could see was black smoke,” he said. “I don’t know how people could stand there for that long.”

Calvin Marr, who lives across town, came to the area for a closer look.

“I’m at Park Avenue East and I could see it,” he said. “You could smell it.”